The present invention relates to a cell-driving-type micro pump member based on the piezoelectric/electrostrictive effect, more specifically, to a cell-driving-type micro pump member having a high response and providing a high pressurizing force. The micro pump includes separate cells whose side walls are made of piezoelectric/electrostrictive elements. Each cell is used as a pressurizing chamber, so that a pressure can be produced in the pressurizing chamber by changing the volume of the cell with the aid of the displacement of the piezoelectric/electrostrictive elements.
Recently, mechanisms providing a change of volume in a pressurizing chamber by deforming a part of the walls forming the pressurizing chamber with the aid of the piezoelectric/electrostrictive effect are known, where the mechanism increases the pressure in a small pressurizing chamber formed in a base part. Such a micro pump member is used, for instance, as an ink pump member or the like in a print head used in an ink jet printer, wherein the pressure in the pressurizing chamber, to which ink is supplied and then stored therein, is increased by the displacement of the piezoelectric/electrostrictive elements, so that the ink particles (droplets) are ejected from nozzle holes connected to the pressurizing chamber, and thus the printing can be carried out.
For instance, in JP-A-6-40030, an example of an ink jet print head as shown in FIG. 16 and FIG. 17 is described, wherein a micro pump member is used as an ink pump member. The ink jet print head 140 is formed by joining an ink nozzle element 142, an ink pump member 144 and a piezoelectric/electrostrictive element 178 to each other, to form a unified body. The ink, which is supplied to ink pressurizing chambers 146 (hereafter simply referred to as pressurizing chambers), is ejected through nozzle holes 154 in the ink nozzle member 142 by the bending deformation of a closing plate 166 (vibration plate) forming the pressurizing chamber 146 in accordance with the deformation of the piezoelectric/electrostrictive element 178, thus inducing a pressure in the pressurizing chamber 146.
The ink pump member 144 is formed as a unified body, in detail, with such a construction that the closing plate 166 and a connecting plate 168, each of which has a planar shape, are superimposed on each other sandwiching a spacer blade 170 therebetween. In the connecting plate 168, first connecting openings 172 and second connecting openings 174 are respectively formed at the positions corresponding to through-holes 156 and orifice holes 158 which are formed in an orifice plate 150 of the ink nozzle element 142. Moreover, a plurality of rectangle-shaped window parts 176 is formed in the spacer plate 170. The spacer plate 170 is superimposed on the connecting plate 168 in such a manner that each of the first connecting openings 172 and second connecting openings 174, which are disposed in the connecting plate 168 is opened to the corresponding window parts 176. In this spacer blade 170, moreover, the closing plate 166 is superimposed on the surface opposite that on which the connecting plate 168 is superimposed, so that the openings of the window parts 176 are closed at the closing plate 166. By so doing, the pressurizing chambers 146, which are connected to the outside via the first and second connecting openings 172, 174, are formed in the inside of the ink pump member 144.
In such an ink jet print head 140, however, there are the following problems. In order to provide a greater displacement so as to be able to eject a greater number of droplets, it is effective to decrease the thickness of the closing plate 166 (vibrating plate) in the ink pump member 144. However, this induces a decrease in the rigidity and reduces the high responsiveness. On the other hand, a significant enhancement in the high responsiveness requires an increase in the rigidity. For this purpose, it would be effective to increase the thickness of the closing plate 166 (vibrating plate), but this treatment provides a reduced displacement, thereby making it impossible to eject the required number of droplets. That is, in the ink pump member, it is difficult to attain both a greater displacement and a higher response property by the bending deformation of the vibrating plate due to the displacement of the piezoelectric/electrostrictive element. This is the first problem.
As for the second problem, it has been found that if one wants to make the adjacent ink pump members the same action, the displacement is reduced compared with the case where the piezoelectric/electrostrictive element is singly driven; this results in failure to display the intrinsic characteristics. That is, the vibrating plates of two adjacent ink pump members are bent simultaneously, so that a pulling force is applied to the walls between the ink pump members, thereby making it difficult to bend the vibrating plates.
Although not shown in the drawings, it has been proposed in JP-A-6-350155 that the interference due to the mutual displacement of the piezoelectric/electrostrictive elements is suppressed by disposing a groove between a concave part (ink pressurizing chamber) and the adjacent concave part, that is, by disposing a groove between adjacent ink pump members.
Moreover, as for a micro pump member based on the known piezoelectric/electrostrictive effect, for instance, a micro pump member, which is driven in shear mode and is similarly used in an ink jet head, is employed. This is a micro pump 271 having such a structure as shown in FIG. 7, wherein a plurality of piezoelectric/electrostrictive elements as comb teeth 276, that is, driving parts 274, are arranged like teeth of a comb on a base plate 272, and cells 273 having substantially rectangular form are formed by a closing slit 275 between the comb teeth with a cover plate 277. The openings at the front end of the micro pump member 271 are closed by a nozzle plate 9 having nozzles 8, so that an ink jet head 270 is formed so as to use the cells 273 as pressurizing chambers. By applying a driving electric field in a direction vertical to the direction of polarizing field in the driving parts 274, that is, comb teeth 276, consisting of the piezoelectric/electrostrictive material, the comb teeth 276 are deformed and thus the volume of the cells 273 are changed, thereby enabling the ink stored in the cells 273 to be ejected. Furthermore, the method of driving where the displacement results from the driving electric field in the direction vertical to the direction of polarization in the piezoelectric/electrostrictive elements is called the shear mode method.
Such a micro pump member 271 is manufactured according to the steps shown in FIG. 8(a)-FIG. 8(e). Firstly, a piezoelectric/electrostrictive material 86 is provided as shown in FIG. 8(a), and fired in FIG. 8(b). In FIG. 8(c), the polarization treatment is carried out and in FIG. 8(d), fine slits are formed with a dicing saw or the like, and driving parts 274 are arranged like the teeth of a comb in a regular form by interposing therebetween a plurality of slits 275 corresponding to respective spaces for storing the ink, and then electrodes are formed on the wall surfaces in the slits 275 in FIG. 8(e). After that, as shown in FIG. 7, the cover plate 277 comprising a glass plate or the like is mounted, and then the openings at the front end of the comb teeth are closed with the nozzle plate 9 having the nozzles 8, so that the cells 273 used as the pressurizing chambers are formed.
In such a manufacturing method, however, there are the following problems due to machining rigid, fired piezoelectric/electrostrictive materials. The first problem is that it is time-consuming to machine the slits with the dicing saw or the like, and therefore it is unsuitable for mass production.
Furthermore, the second problem is the cost increase. This is because sufficient cleaning is required after machining since the products are polluted with the free grinder particles for processing and the process liquid; this would require complex cleaning processes to clean them in a satisfactory manner due to the reduced strength after machining, with necessarily accompanying the process for drying, and facilities for both treating water for cleaning and exhausted water and the management thereof as well.
The third problem is that the slits forming the cells used as pressurizing chambers are restricted by the thickness of the dicing blade used for machining, so that a width of approximately 60 μm or less cannot be realized. Additionally, the thickness of the comb teeth, that is, the driving parts, also has a limitation regarding the depth of the slits so that the required grinding force for the dicing blade is obtained, and it is difficult to form cells or pressurizing chambers having a high aspect ratio of, for example, 10 or more, so that it is difficult to obtain a high power micro pump member having a high density or a high strength.
Generally, the aspect ratio is denoted by the ratio of the diameter to the axial length, in the case of an aperture having a cylindrical form. If the aperture has a non-cylindrical form, such as that shown in FIG. 8(d), i.e., in the case of the slit 275, which is later closed and thus becomes a cell (pressurizing chamber), the aspect ratio is denoted by the ratio of the shortest spacing between two comb teeth forming the slit 275, the comb teeth facing each other, that is, the width of the slit 275 to the depth of the slit 275. A pressurizing chamber having a high aspect ratio implies a pressurizing chamber whose height is greater compared with the inside width.
The fourth problem is that machining with a dicing blade only allows the production of straight and flat slits, so that a subsequent process for adhering parts is required if one wants the cells (the pressurizing chamber), to have a complex form. Moreover, since electric stress deformation rises up to the joint end of the slit plates when activated as a result of the straight machining, the durability of the joint surfaces is liable to be reduced therefrom.
The fifth problem is that, since the slit is formed with the grinding process after firing, micro cracks and fractures inside the grains of the piezoelectric/electrostrictive particles often occur at the side surface of the comb-like driving parts 274, and the characteristics of the cells are liable to be deteriorated. FIG. 9(a) and FIG. 9(b) are drawings illustrating this fact: FIG. 9(a) is a side view from Q in FIG. 8(d) and FIG. 9(b) is a magnified section of part N in FIG. 9(a). In the case of the grinding process with the dicing saw, either micro cracks from the machining or particles fractured in the grains result, on the side surface of comb-like driving parts 274 (teeth 276 of a comb), the particles thus have deteriorating properties, so that when the cell is driven, the performance inherent in the material cannot be obtained, and the micro cracks propagate, thereby damaging the device itself.
In the conventional micro pump member 271, moreover, problems occur as a result of the driving in the shear mode. The sixth problem is that after firing and carrying out the polarizing treatment, manufacturing processes involving heating to a temperature of the Curie temperature or higher caused the polarization in the piezoelectric/electrostrictive material and cannot be applied. As a result, in the case of fixing/wiring the micro pump members to a circuit board, the soldering process using the reflow-soldering method or adhesion under heating cannot be applied, due to the thermal restriction, and the throughput is reduced, thereby increasing the cost of manufacturing. Moreover, either laser machining or machining generating heat is also restricted.
Moreover, as for the seventh problem, it is noted that since the driving electric field is generated in the direction vertical to the direction of polarization field, activation with a high field strength, which causes the change in the state of polarization, is not permitted, so that a greater amount of strain cannot be obtained. If, however, a high driving electric field is generated, the state of polarization gradually changes during the driving period, hence, similarly reducing the amount of strain. As a result, the basic performance of the micro pump member deteriorates.
Moreover, in the conventional micro pump member 271, problems occur as a result of the structure in which the base plate, driving parts and cover plate are unified into one body, along with the problems which occur as a result of the above-mentioned manufacturing method, that is, the problem due to driving in the shear mode.
The eighth problem is the difficulty in making the adjacent cells (pressurizing chambers) in the same action. FIG. 15 is a sectional view showing the states of deactivation and activation for the micro pump member 271 as an example. When the driving electric field is applied, that is, in the case of the OFF state, the driving parts 274 of the piezoelectric/electrostrictive elements are not deformed, whereas when the driving electric field is applied to a specified driving part 274, that is, in the case of the ON state, the driving part 274 is deformed. As can be seen in FIG. 15, the driving part 274 acts as the driving elements for two cells 273, so that when the volume of the one cell increases, the volume of the adjacent cells decreases. When, for instance, the micro pump member 271 is used as the ink jet head 270 shown in FIG. 7, ink cannot be simultaneously ejected from the adjacent cells, that is, the pressurizing chambers. As a result, at least two actions are required to spray ink to an article to be sprayed in the minimum spacing between the ink jet head and the article. This is undesirable in view of enhancing speed in the ink ejecting process.